Method for block polymer synthesis by controlled radical polymerisation in the presence of a disulphide compound

ABSTRACT

The invention concerns a method for preparing a first generation polymer comprising a step which consists in radical polymerization of a composition containing: at least an ethylenically unsaturated monomer a free radical source, and at least a disulfide compound.

[0001] The present invention relates to a novel method for free-radicalpolymerization which provides block polymers.

[0002] Block polymers are conventionally prepared by ionicpolymerization. This type of polymerization has the drawback of allowingthe polymerization of only certain types of apolar monomer, inparticular styrene and butadiene, and of requiring a particularly purereaction medium and temperatures often below ambient temperature so asto minimize parasitic reactions, hence severe operational constraints.

[0003] Free-radical polymerization has the advantage of being easilycarried out without having to comply with excessive purity conditions,and at temperatures greater than or equal to ambient temperature.However, until recently, no method for free-radical polymerizationexisted for obtaining block polymers.

[0004] In conventional free-radical polymerization, the growingmicroradicals have nonselective reactivity: the chains terminateirreversibly by coupling or dismutation. Consequently, it is verydifficult to control the structure of the chains. The possibilities ofobtaining functional telechelic polymers or block copolymers are verylimited. Recently, a new method for free-radical polymerization has beendeveloped: this is “controlled” or “living” free-radical polymerization.Several techniques have been developed, in which the ends of polymerchains can be reactivated by virtue of a reversible termination ortransfer reaction (dormant species/active species equilibrium).

[0005] Controlled free-radical polymerization has the followingdistinctive characteristics:

[0006] 1. the number of chains is fixed throughout the duration of thereaction,

[0007] 2. the chains all grow at the same rate, resulting in:

[0008] a linear increase in the molecular masses with conversion,

[0009] a narrow distribution of masses,

[0010] 3. the average molecular mass is controlled by themonomer/chain-precursor molar ratio,

[0011] 4. the possibility of preparing block copolymers.

[0012] The controlled nature is all the more pronounced when the rate ofreactivation of the chains to radical is very great in view of the rateof growth of the chains (propagation). There are cases where this is notalways true (i.e. the rate of reactivation of the chains to radical isless than the rate of propagation) and conditions 1 and 2 are notobserved, nevertheless it is always possible to prepare blockcopolymers.

[0013] Recently, methods for living free-radical polymerization bythermal initiation have been developed. For example, PCT patentapplications WO 98/01478 in the name of Dupont de Nemours and WO99/35178 in the name of Rhodia Chimie describe the use of agents forreversible transfer by addition-fragmentation of the RSC═SR′ dithioestertype, for the synthesis of copolymers with controlled architecture.Another family of reversible transfer agents, RSC═SOR′ xanthates, hasbeen described in patent application WO 98/58974 from the company RhodiaChimie, as precursors for block copolymers. The control of free-radicalpolymerization with RS(C═S)NR₁R₂ dithiocarbamates has also recently beendescribed in patent applications WO 99/35177 in the name of Rhodia andWO 99/31144 in the name of Dupont de Nemours.

[0014] Controlled free-radical polymerization has an advantage overconventional free-radical polymerization when it comes to preparingfunctionalized low molecular weight chains (reactive telomers). Suchpolymers are desired for specific applications such as, for example,coatings and adhesives.

[0015] Thus, when seeking to synthesize chains grafted with on average 2functional comonomers, the fraction of chains with at most onefunctional site becomes great when the average degree of polymerizationis less than the threshold value (e.g. 20 or 30). Controlledfree-radical polymerization itself makes it possible to reduce, or eveninhibit, the formation of these oligomers with zero or one functionalsite, which degrade the performance in terms of application.

[0016] In the remainder of the description, the term “polymer” is usedto describe homopolymers or copolymers, unless otherwise indicated.

[0017] In addition, the term “block polymer” is intended to mean acopolymer comprising at least two series of blocks of monomer units withdifferent chemical constitutions. The blocks may consist of ahomopolymer or of a polymer obtained from a mixture of ethylenicallyunsaturated monomers. In this case, the block may be a random copolymer.The block copolymer may comprise two blocks, each consisting of randomcopolymers. In this case, the ethylenically unsaturated monomers aresuch that the blocks obtained are different in nature. The expression“different in nature” is intended to mean blocks consisting of monomersof different types, but also blocks consisting of monomers of the sametype but in different amounts.

[0018] An aim of the present invention is to provide a novel method forfree-radical polymerization which exhibits improved effectiveness andbetter control of the formation of the polymer.

[0019] This aim, and others which will become apparent on reading thedescription, are achieved by the present invention, which relates to amethod for preparing a first generation polymer, which comprises a stepof free-radical polymerization of a composition comprising:

[0020] at least one ethylenically unsaturated monomer,

[0021] a source of free radicals,

[0022] at least one compound (I) of general formula (IA) or (IB):

[0023] a compound of formula (II)

[0024] in which

[0025] R¹ represents a group chosen from alkyl, acyl, aryl, aralkyl,alkene or alkyne groups, saturated, unsaturated or aromatic carbonaceousrings or heterocycles, or a polymer chain,

[0026] Z and Z₁, which may be identical or different, represent agroup-chosen from

[0027] alkyl, acyl, aryl, aralkyl, alkene or alkyne groups, orsaturated, unsaturated or aromatic carbonaceous rings or heterocycles,which may be substituted,

[0028] the group —OR² in which R² is an alkyl, acyl, aryl, aralkyl,alkene or alkyne group, a saturated, unsaturated or aromaticcarbonaceous ring or heterocycle, a polymer chain, —CH₂C_(n)F_(2n+1)with n between 1 and 20, or a group —CR⁵R⁶PO(OR⁷)₂ in which R⁵ and R⁶are each separately a hydrogen atom, a halogen, an alkyl group, aheterocyclic group, a group —NO₂, —SO₃R₈, —NCO, CN, R⁸, —OR⁸, —SR⁸, —NR⁸₂, —COOR⁸, O₂CR⁸, —CONR⁸ ₂, —NCOR⁸ ₂, or C_(n)F_(2n+1) with n between 1and 20, R⁸, which may be identical or different, being chosen from agroup consisting of the following groups: alkyl, alkenyl, alkynyl,cycloalkenyl, cycloalkynyl, aryl, optionally condensed with an aromaticor nonaromatic heterocycle, alkaryl, aralkyl and heteroaryl, R⁸ possiblybeing substituted with one or more groups, which may be identical ordifferent, chosen from halogen, ═O, ═S, OH, alkoxy, SH, thioalkoxy, NH₂,mono- or dialkylamino, CN, COOH, ester, amide, and C_(n)F_(2n+1) with nbetween 1 and 20, and/or optionally interrupted with one or more atomschosen from O, S, N and P, or R⁵ and R⁶ form, together with the carbonatom to which they are attached, a group ═O or ═S or a hydrocarbon-basedring or a heterocycle, and R⁷, which may be identical or different,represents a group R⁸ as defined above or they together form a C₂-C₄hydrocarbon-based chain optionally interrupted with a hetero atom chosenfrom O, S and N;

[0029] the group —NR³R⁴ in which R³ and R⁴, which may be identical ordifferent, are chosen from optionally substituted alkyl, acyl, aryl,aralkyl, alkene or alkyne groups, and saturated, unsaturated or aromaticcarbonaceous rings or heterocycles, which may be substituted, and R³ andR⁴ together form an optionally substituted ring containing at least 5members, with the additional condition that R³ and R⁴ induce adelocalizing or electron-withdrawing effect with respect to the electrondensity of the nitrogen atom,

[0030] p is an integer greater than or equal to 1.

[0031] The invention also relates to a method for preparing a blockpolymer, and to the compositions which can be obtained using one orother of the methods of the invention.

[0032] An advantage of the method of the invention is that it makes itpossible, for relatively inactive monomer-compound (I) systems, torender virtually linear the evolution of the molecular mass (Mn) as afunction of the monomer conversion rate. It is thus possible to obtain,for low conversion rates, low molecular masses without, however,increasing the compound (I)/monomer molar ratio.

[0033] Another advantage is that compound (II) which is of use in themethod of the invention makes it possible to accelerate the consumptionof compound (I) undergoing reaction. This method is particularlyadvantageous when compound (I) exhibits low reactivity with respect tothe monomer used.

[0034] Specifically, for relatively inactive monomer-compound (I)systems, the resulting polymer composition contains a residual amount ofcompound (I). The presence of this residual amount of compound (I)disturbs the subsequent formation of a block copolymer. It is thereforeparticularly advantageous to have a method of synthesis which makes itpossible to reduce the residual amount of compound (I) in the polymercomposition obtained.

[0035] The groups R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, Z and Z¹ may be linearor branched, substituted or unsubstituted groups. The substituents maybe chosen from phenyl groups, aromatic groups, saturated or unsaturatedcarbonaceous rings, saturated or unsaturated heterocycles, or groupsconsisting of the following groups: alkoxycarbonyl or aryloxycarbonyl(—COOR), carboxy (—COOH), acyloxy (—O₂CR), carbamoyl (—CONR₂), cyano(—CN), alkylcarbonyl, alkylarylcarbonyl, arylcarbonyl,arylalkylcarbonyl, phthalimido, maleimido, succinimido, amidino,guanidimo, hydroxyl (—OH), amino (—NR₂), halogen, perfluoroalkylC_(n)F_(2n+1), allyl, epoxy, alkoxy (—OR), S-alkyl or S-aryl, or groupsexhibiting a hydrophilic or ionic nature, such as alkali metal salts ofcarboxylic acids, alkali metals salts of sulfonic acid, poly(alkyleneoxide) (PEO, PPO) chains or cationic substituents (quaternary ammonium),R representing an alkyl or aryl group, or a polymer chain.

[0036] In the compound (I) of general formula (IA) or (IB), p ispreferably between 1 and 10, preferably between 1 and 5. When p isgreater than 1, then the groups R¹ and/or Z may be identical ordifferent.

[0037] According to a particular embodiment, R¹ is an alkyl group,preferably substituted. The group R¹ is, for example, chosen from thefollowing substituted alkyl groups:

[0038] CH₂C₆H₅

[0039] CH (CH₃)(CO₂Et)

[0040] CH(CH3)CO2Me)

[0041] CH (CH₃)(C₆H₅)

[0042] CH (CO₂Et)₂

[0043] C (CH₃)(CO₂Et)(S—C₆H₅)

[0044] C (CH₃)₂(C₆H₅)

[0045] C (CH₃)₂CN

[0046] in which Et represents an ethyl group, Me a methyl group and Ph aphenyl group.

[0047] When R¹ is a polymer chain, this polymer chain may be derivedfrom a free-radical polymerization or ionic polymerization or derivedfrom a polycondensation.

[0048] According to a particular embodiment, the compound (I) is suchthat the group Z is chosen from the groups consisting of the followinggroups: alkyl, —OR², R² being an alkyl group comprising from 1 to 20carbon atoms, an aryl group, an aralkyl group or a group—CH₂C_(n)F_(2n+1) with n between 1 and 20; —NR³R⁴ in which R³ and R⁴,which may be identical or different, are an alkyl group comprising from1 to 20 carbon atoms.

[0049] According to a particularly preferred embodiment, the compounds(I) exhibit a reactivity with respect to the monomer such that thecompound (I) exhibits a transfer constant (Ctr) of less than 10 withrespect to the monomer.

[0050] The transfer constant (Ctr) is defined as the ratio of the rateconstants for transfer and for propagation at zero conversion ofcompound (I) and of monomer. The transfer constant can be measured byMayo's law as described in J. Am. Chem. Soc., 65, 2324 (1943).

[0051] In the method of the present invention, the transfert constantfor the compound (I) with respect to the monomer is preferably lessthan 1. Such compounds are, for example, monomer/compound (I) systemssuch as the methyl methacrylate/S-propionyl O-ethyl xanthate system orthe styrene/S-benzyl O-ethyl xanthate system.

[0052] The compounds (I) are readily accessible. Those in which Z is—OR² in which R² is an alkyl group, termed alkyl xanthates, can inparticular be obtained by reaction between a xanthate salt, such as analkali metal salt of the type:

[0053] and a halogenated derivative of the type: Hal-R¹ with Hal chosenfrom Cl, Br and I.

[0054] The compounds (I) can also be obtained by the method in which:

[0055] a disulfide compound (S) of formula (A):

[0056] and a diazo compound of formula:

R ¹ —N═N—R ¹

[0057] in which R¹ and Z are as defined above, are mixed and heated.

[0058] The amount of compound (I) depends on the molecular mass of thedesired polymer according to the lawMn_(theoretical)=[M]₀/[I]₀×conversion rate+MW(I) in which M₀ is theinitial concentration of monomers, [I]₀ is the initial concentration ofcompound (I) and MW(I) the molecular mass of the compound (I).

[0059] The compound (II) of use is such that the groups Z₁ are identicalor different. According to a particular embodiment, Z₁ is —OR² in whichR² is an alkyl radical. For example, R2 is a linear or branched,substituted or unsubstituted methyl, ethyl, propyl, etc. group. Z¹ maybe —OC₂H₅ or —OCH₂CF₃.

[0060] The methods for preparing these compounds (II) are widelydescribed in the literature. These compounds may be obtained byoxidation of the corresponding xanthate salt Z¹(C═S)S⁻M⁺ with iodine.

[0061] The amount of compound (II) in the composition of use in thecontext of the invention may be very variable; however, an amount ofcompounds (II) of between 0.1 and 20 mol % relative to the number ofmoles of compound (I) is preferred. This amount is preferably between 1and 10%.

[0062] The method of the invention is, in all cases, carried out in thepresence of a source of free radicals, these free radicals being able tobe generated by the monomer itself or by the compound (II) under thepolymerization conditions. Specifically, for certain monomers, such asstyrene, the free radicals making it possible to initiate thepolymerization can be generated by the ethylenically unsaturated monomeritself at sufficiently high temperatures, generally above 100° C. Inthis case, it is not necessary to add a source of additional freeradicals. The same is true with the compounds (II).

[0063] The source of free radicals of use in the method of the presentinvention is generally a free-radical polymerization initiator. Thefree-radical polymerization initiator can be chosen from the initiatorsconventionally used in free-radical polymerization. It may, for example,be one of the following initiators:

[0064] hydrogen peroxides such as: tert-butyl hydroperoxide, cumenehydroperoxide, tert-butyl peroxyacetate, tert-butyl peroxybenzoate,tert-butyl peroxyoctoate, tert-butyl peroxyneodecanoate, tert-butylperoxyisobutyrate, lauroyl peroxide, tert-amyl peroxypivalate,tert-butyl peroxypivalate, dicumyl peroxide, benzoyl peroxide, potassiumpersulfate or ammonium persulfate,

[0065] azo compounds such as: 2,2′-azobis(isobutyronitrile),2,2′-azobis(2-butanenitrile), 4,4′-azobis(4-pentanoic acid),1,1′-azobis(cyclohexane-carbonitrile), 2-(tert-butylazo)-2-cyanopropane,2,2′-azobis[2-methyl-N-(1,1)-bis(hydroxymethyl)-2-hydroxyethyl]propionamide,2,2′-azobis(2-methyl-N-hydroxyethyl]propionamide,2,2′-azobis(N,N′-dimethylene-isobutyramidine)dichloride,2,2′-azobis(2-amidinopropane)dichloride,2,2′-azobis(N,N-dimethyleneisobutyramide),2,2′-azobis(2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyehtyl]propionamide),2,2′-azobis(2-methyl-N-[1,1-bis(hydroxymethyl)ethyl]propionamide),2,2′-azobis[2-methyl-N-(2-hydroxyethyl)propionamide) or2,2′-azobis(isobutyramide)dihydrate,

[0066] redox systems comprising combinations such as:

[0067] mixtures of hydrogen peroxide or alkyl peroxide, peresters,percarbonates and the like and of any one of the salts of iron, titanoussalts, zinc formaldehyde sulfoxylate or sodium formaldehyde sulfoxylate,and reducing sugars;

[0068] alkali metal or ammonium persulfates, perborates or perchloratesin combination with an alkali metal bisulfite, such as sodiummetabisulfite, and reducing sugars;

[0069] alkali metal persulfates in combination with an arylphosphinicacid, such as benzenephosphonic acid and other similar acids, andreducing sugars.

[0070] According to one embodiment, the amount of initiator to be usedis determined in such a way that the amount of radicals generated is atmost 50 mol % relative to the amount of compound (III), preferably atmost 20 mol %.

[0071] The ethylenically unsaturated monomers of use in the method ofthe present invention are all monomers which polymerize in the presenceof the compounds (I) and (II), to give active polymer chains.

[0072] These ethylenically unsaturated monomers are, for example:

[0073] styrene and styrene derivatives such as alpha-methylstyrene orvinyltoluene

[0074] vinyl esters of carboxylic acid, such as vinyl acetate, vinylVersatate® or vinyl propionate,

[0075] vinyl halides,

[0076] ethylenically unsaturated monocarboxylic and dicarboxylic acids,such as acrylic acid, methacrylic acid, itaconic acid, maleic acid orfumaric acid, and monoalkyl esters of dicarboxylic acids of the typementioned with alkanols preferably having from 1 to 4 carbon atoms andtheir N-substituted derivatives,

[0077] amides of unsaturated carboxylic acids, such as acrylamide,methacrylamide, N-methylolacrylamide or methacrylamide, orN-alkylacrylamides,

[0078] ethylenic monomers comprising a sulfonic acid group and itsammonium or alkali metal salts, for example vinylsulfonic acid,vinylbenzenesulfonic acid, alpha-acrylamidomethylpropanesulfonic acidand 2-sulfoethylene methacrylate,

[0079] amides of vinylamine, in particular vinylformamide orvinylacetamide,

[0080] unsaturated ethylenic monomers comprising a secondary, tertiaryor quaternary amino group, or a heterocyclic group containing nitrogen,such as, for example, vinylpyridines, vinylimidazole, aminoalkyl(meth)acrylates and aminoalkyl (meth)acrylamides such asdimethylaminoethyl acrylate or methacrylate, di-tert-butylaminoethylacrylate or methacrylate, or dimethylamino methylacrylamide or

[0081] methacrylamide, or zwitterionic monomers such as, for example,sulfopropyl-(dimethyl)aminopropyl acrylate,

[0082] dienes, for example butadiene and chloroprene,

[0083] (meth)acrylic esters,

[0084] vinyl nitriles,

[0085] vinylphosphonic acid and derivatives thereof.

[0086] The term “(meth)acrylic esters” denotes esters of acrylic acidand of methacrylic acid with hydrogenated or fluorinated C₁-C₁₂alcohols, preferably C₁-C₈ alcohols. Among the compounds of this type,mentioned may be of: methyl acrylate, ethyl acrylate, propyl acrylate,n-butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, tert-butylacrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylateand isobutyl methacrylate.

[0087] The vinyl nitriles include more particularly those having from 3to 12 carbon atoms, such as in particular acrylonitrile andmethacrylonitrile.

[0088] For the preparation of a polyvinylamine block, use is preferablymade, as ethylenically unsaturated monomers, of amides of vinylamine,for example vinylformamide or vinylacetamide. The polymer obtained isthen hydrolyzed at acid or basic pH.

[0089] For the preparation of a poly(vinyl alcohol) block, use ispreferably made, as ethylenically unsaturated monomers, of vinyl estersof carboxylic acid, such as, for example, vinyl acetate. The polymerobtained is then hydrolyzed at acid or basic pH.

[0090] The types and amounts of polymerizable monomers used according tothe present invention vary as a function of the specific finalapplication for which the polymer is intended. These variations are wellknown and can be readily determined by those skilled in the art.

[0091] These ethylenically unsaturated monomers may be used alone or asmixtures.

[0092] According to a particular embodiment, in the method for preparinga first generation polymer, the ethylenically unsaturated monomercorresponds to the formula CXX′(═CV—CV′)_(b)═CH₂ in which

[0093] V and V′, which may be identical or different, represent: H, analkyl group or a halogen,

[0094] X and X′, which may be identical or different, represent H, ahalogen or a group R⁴, OR⁴, O₂COR₄, NHCOH, OH, NH₂, NHR₄, N(R⁴)₂,(R⁴)₂N⁺O⁻, NHCOR⁴, CO₂H, CO₂R⁴, CN, CONH₂, CONHR⁴ or CON(R⁴)₂, in whichR⁴ is chosen from alkyl, aryl, aralkyl, alkaryl, alkene or organosilylgroups which are optionally perfluorinated and optionally substitutedwith one or more carboxy, epoxy, hydroxyl, alkoxy, amino, halogen orsulfonic groups, and

[0095] b is 0 or 1.

[0096] The polymerization may be carried out in bulk, in solution, underemulsion conditions, in dispersion or in suspension. It is preferablycarried out in solution or under emulsion conditions.

[0097] The method is preferably carried out semi-continuously.

[0098] The compound (II) may be present in the polymerization mediumfrom the start of the reaction. It may also be added during thepolymerization.

[0099] The temperature may range between ambient temperature and 150°C., depending on the nature of the monomers used.

[0100] In general, during the polymerization, the immediate content ofpolymer relative to the immediate amount of monomer and polymer isbetween 50 and 99% by weight, and preferably between 75 and 99%, evenmore preferentially between 90 and 99%. This content is maintained, in aknown manner, by controlling the temperature and the rate of addition ofthe reagents and, optionally, of the polymerization initiator.

[0101] Generally, the method is carried out in the absence of a UVsource, by thermal initiation.

[0102] The method of the invention can be carried out using a mixture ofethylenically unsaturated monomers. In this case, a random firstgeneration polymer is obtained. By selecting monomers of specificnature, for example hydrophilic monomers and hydrophobic monomers, andthe amount of each of these monomers in the block, a block is obtainedwhich has particular properties. This procedure is particularlyadvantageous when the first generation polymer thus obtained is anintermediate in the preparation of a block copolymer.

[0103] Using the composition of the present invention containing acompound (I) of formula (IA) or (IB) with p equal to I, andethylenically unsaturated monomers corresponding to the formulaCXX′(═CV—CV′)_(b)═CH₂, defined above, a first generation polymer isobtained of formula (P1):

[0104] with n greater than or equal to 1, Z, X, X′, V, V′, b and R¹being as defined above.

[0105] The present invention also relates to a method for preparing anNth generation block copolymer by free-radical polymerization, N beinggreater than or equal to 2, which comprises:

[0106] a first step of free-radical polymerization obtained using acomposition comprising

[0107] at least one ethylenically unsaturated monomer,

[0108] a source of free radicals,

[0109] at least one compound (I) of general formula (IA) or (IB),

[0110] a number N-1 of free-radical polymerization steps, each of thesesteps being carried out using a composition comprising:

[0111] at least one ethylenically unsaturated monomer,

[0112] a source of free radicals, and

[0113] the polymer obtained in the preceding polymerization step,

[0114] the ethylenically unsaturated monomer(s) in each of the stepsbeing such that the block formed in a step is different in nature fromthe block formed in the preceding step, and the first polymerizationstep and/or the subsequent polymerization steps are carried out in thepresence of at least one compound of formula (II).

[0115] For example, a second generation block copolymer can be obtainedby a method which comprises the free-radical polymerization of acomposition comprising:

[0116] at least one ethylenically unsaturated monomer,

[0117] a source of free radicals, and

[0118] the first generation polymer obtained by free-radicalpolymerization of the composition containing a source of free radicals,the compounds (I) and (II) and an ethylenically unsaturated monomer,

[0119] the ethylenically unsaturated monomers which allow the secondblock to be obtained being such that the block is different in naturefrom the first generation polymer.

[0120] According to one embodiment of the invention, (1) a firstgeneration polymer is synthesized using a composition comprising one ormore ethylenically unsaturated monomers, a source of free radicals, acompound of formula (IA) and/or (IB) and a compound (II), and then (2)the first generation polymer obtained in step (1) is used to prepare a(second generation) diblock copolymer by bringing this first generationpolymer into contact with one or more ethylenically unsaturated monomersand a source of free radicals, the block obtained in step (2) beingdifferent in nature from the first generation polymer of step (1).According to a particular embodiment, step (2) can be carried out in thepresence of the compound (II).

[0121] This step (2) can be repeated with further monomers and thediblock copolymer obtained, so as to synthesize a new block and obtain atriblock copolymer.

[0122] It is thus possible to repeat as many times as necessary thepolymerization step using a block copolymer so as to obtain a copolymerwith an additional block.

[0123] The method of the invention makes it possible in particular toobtain a diblock copolymer of general formula (P2):

[0124] using a composition containing a source of free radicals, apolymer (P1) as defined above and an ethylenically unsaturated monomerof formula

CYY′(═CW—CW′)_(a) ═CH ₂,

[0125] in which

[0126] n and m, which may be identical or different, are greater than orequal to 1,

[0127] W and W′, which may be identical or different, represent: H, analkyl group or a halogen,

[0128] Y and Y′, which may be identical or different, represent H, ahalogen or a group R⁴, OR⁴, O₂COR⁴, NHCOH, OH, NH₂, NHR⁴, N(R⁴)₂,(R⁴)₂N⁺O⁻, NHCOR⁴, CO₂H, CO₂R⁴, CN, CONH₂, CONHR⁴ or CON(R⁴)₂, in whichR⁴ is chosen from alkyl, aryl, aralkyl, alkaryl, alkene or organosilylgroups which are optionally perfluorinated and optionally substitutedwith one or more carboxy, epoxy, hydroxyl, alkoxy, amino, halogen orsulfonic groups, and

[0129] a and b, which may be identical or different, equal 0 or 1,

[0130] V, V′, X, X′, Z, Z¹ and R¹ are as defined above.

[0131] The ethylenically unsaturated monomers which are of use are thosedescribed above.

[0132] The method above is described based on a polymer (P1) obtainedusing a compound (I) with p equal to 1; however, this teaching isdirectly applicable to the polymers obtained using the compounds (I) offormula (IA) and/or (IB) with p greater than 1.

[0133] The compounds of formula (IA) and (IB) when p is greater than 1are particularly advantageous since they make it possible to increase apolymer chain on at least two active sites. With this type of compound,it is possible to economize on the polymerization steps to obtain acopolymer comprising n blocks. Thus, if p equals 2 in the formula (IA)or (IB), the first block is obtained by polymerization of a monomer M1in the presence of the compound of formula (IA) or (IB). This firstblock can then grow at each of its ends by polymerization of a secondmonomer M2. A triblock copolymer is obtained, and this triblockcopolymer can, itself, grow at each of its ends by polymerization of athird monomer M3. Thus, a “pentablock” copolymer is obtained in onlythree steps. If p is greater than 2, the method makes it possible toobtain block copolymers or homopolymers with a “multiarm” or starstructure.

[0134] According to this method for preparing block polymers, when it isdesired to obtain polymers comprising blocks which are homogeneous andnot comprising a composition gradient, and if all the successivepolymerizations are carried out in the same reactor, it is essential forall the monomers used during a step to have been consumed before thepolymerization of the subsequent step begins, and therefore before thenew monomers are introduced.

[0135] When it is desired to obtain a random block, the polymerizationstep is carried out with a composition containing a mixture ofethylenically unsaturated monomers.

[0136] The polymers obtained according to the method of the inventionexhibit a low polydispersity index and a controlled molecular mass. Thepolydispersity index is at most 2, preferably at most 1.5.

[0137] According to a particular embodiment, the block polymers compriseat least two polymer blocks chosen from the following combinations:

[0138] polystyrene/poly(methyl acrylate),

[0139] polystyrene/poly(ethyl acrylate),

[0140] polystyrene/poly(tert-butyl acrylate),

[0141] poly(ethyl acrylate)/poly(vinyl acetate),

[0142] poly(butyl acrylate)/poly(vinyl acetate),

[0143] poly(tert-butyl acrylate)/poly(vinyl acetate).

[0144] One of the blocks may also consist of a random copolymer obtainedusing a mixture of ethylenically unsaturated monomers.

[0145] According to a particular embodiment, a diblock copolymercontaining a homopolymer block and a second, random copolymer block isprepared, in a single step, by introducing a second monomer into thereaction medium while the first monomer has not been completelyconsumed, the addition of the second monomer being carried out aftercomplete consumption of the compound (I).

[0146] This embodiment is particularly advantageous with the method ofthe invention which makes it possible, by virtue of the presence of thecompound (II), to increase the rate of consumption of the compound (I).

[0147] According to a particular embodiment, the reactive ends of theblock polymer can be cleaved to form a polymer whose ends are thiols orhydrogen atoms. These modifications can be carried out by reductionand/or hydrolysis.

[0148] The following examples illustrate the invention without, however,limiting the scope thereof.

EXAMPLES

[0149] In the subsequent examples, the molecular mass M_(n) is measuredby gel permeation chromatography (GPC). a—the theoretical number−averagemass M_(n th) is equal to ([M]₀/[A]₀)×conversion+MW(A), in which [M]₀ isthe initial concentration of monomer, [A]₀ is the initial concentrationof xanthate A, MW(A) is the molecular mass of A. M_(n th) describes acomplete consumption of the xanthate A at the start of the reaction andan insignificant contribution of the chains initiated by thefree-radical initiator.

Example 1

[0150] Synthesis of S-propionyl O-ethyl xanthate CH₃CH(CO₂CH₃)S(C═S)OEt(A)

[0151] 5 g (0.04 mol) of methyl chloropropionate CH₃CH(CO₂CH₃)Cl aredissolved in 10 ml of acetone, and 7.2 g (0.045 mol) of potassium saltof xanthic acid EtO(C═S)S⁻K⁺ are added portionwise over 30 minutes. Thereaction mixture is stirred for 1 hour. The product is then extractedwith ether, and washed with an aqueous NaCl solution and then withwater. The product is then dried over MgSO₄ and the solvent isevaporated off under vacuum. 7.5 g (88%) of product A are obtained.

Example 2

[0152] Synthesis of S-propionyl O-trifluoroethyl XanthateCH₃CH(CO₂CH₃)S(C═S)OCH₂CF₃ (B)

[0153] 2 g (20 mmol) of trifluoroethanol in solution in 40 ml of DMF areplaced in a glass round-bottomed flask. 2.4 ml (40 mmol) of CS₂ areadded. The solution is cooled to 0° C., and then 0.96 g (20 mmol) of NaHis added. After stirring for 1 hour at 0° C., 2.34 ml (18 mmol) of2-ethyl bromopropionate are added. The solution is stirred for 1 hour at0° C., and then for two hours at ambient temperature, before beingdiluted with ethyl ether. It is then washed with water and then withbrine. The organic phase is concentrated under vacuum, and then thereaction crude is column purified (9/1:heptane/ethyl acetate). 3.4 g(69% yield) of xanthate B is isolated.

Example 3

[0154] Synthesis of bis-O-ethyl xanthate (D_(A))

[0155] 3 g (0.019 mol) of potassium salt of xanthic acid EtO(C═S)S⁻K⁺are dissolved in 30 ml of anhydrous THF. 2.4 g (0.0095 mol) of I₂ arethen added portionwise at 0° C. The reaction medium is stirred for 2hours and the product is extracted with ether, and washed with asolution of Na₂SO₄, then with an aqueous NaCl solution, and then withwater. After drying with MgSO₄ and evaporation of the solvent undervacuum, 2.1 g (93%) of bis-O-ethyl xanthate are isolated.

Example 4

[0156] Synthesis of bis-O-trifluoroethyl xanthate (D_(B))

[0157] 3 g (0.014 mol) of CF₃CH₂O(C═S)S⁻K⁺ are dissolved in 30 ml ofanhydrous THF. 1.96 g (0.007 mol) of I₂ are then added portionwise at 0°C. The reaction medium is stirred for 2 hours and the product isextracted with ether, and washed with a solution of Na₂SO₄, then with anaqueous NaCl solution, and then with water. After drying with MgSO₄ andevaporation of the solvent under vacuum, 1.54 g of pure product (63%)are harvested by column chromatography (eluent: heptane).

Example 5

[0158] Polymerization of Ethyl Acrylate in the Presence of A:

[0159] 0.146 g(0.7 mmol) of A (S-propionyl O-ethyl xanthate), 3.4 mg(0.02 mmol) of AIBN (azobisisobutyro-nitrile) and 5.5 g (55 mmol) ofethyl acrylate are dissolved in 6 ml of toluene. Fractions of this stocksolution are distributed into Carius tubes. The content of these tubesis degassed by three successive cycles of “freezing-vacuum-return toambient temperature”. The tubes are then flame-sealed under vacuum. Thetubes are then placed in a thermostatic bath at 80° C. They arewithdrawn at regular time intervals, then opened and their content isanalyzed.

[0160] The table below gives the evolution of the number-average molarmass (M_(n)) and also the polymolecularity index (M_(w)/M_(n)) as afunction of the monomer conversion. Conversion (%) M_(n) (g/mol)M_(n th) (g/mol)^(a) M_(w)/M_(n) 1 4.4 5 130   560 1.71 2 11.6 5 710 1140 1.81 3 18.2 6 060 1 670 1.70 4 41 6 745 3 490 1.72 5 96.5 7 290 7940 1.77

Example 6

[0161] Polymerization of Ethyl Acrylate in the Presence of A and ofD_(A) (2%)

[0162] The procedure of example 5 is repeated, in which 3.41 mg (0.014mmol) of bis-O-ethyl xanthate D_(A) are added to the stock solution (2mol % relative to compound A). The results obtained are given in thetable below: Conversion (%) M_(n) (g/mol) M_(n th) (g/mol)^(a)M_(w)/M_(n) 1 4.1 3 600   540 2.37 2 10.2 4 000 1 020 2.14 3 14.4 4 1001 360 2.25 4 45.7 5 200 3 870 2.11 5 76.7 7 050 6 350 1.89

Example 7

[0163] Polymerization of Ethyl Acrylate in the Presence of A and ofD_(A) (5%)

[0164] The procedure of example 5 is repeated, in which 8.53 mg (0.035mmol) of bis-O-ethyl xanthate D_(A) are added to the stock solution (5mol % relative to xanthate A). The results are given in the table below:Conversion (%) M_(n) (g/mol) M_(n th) (g/mol)^(a) M_(w)/M_(n) 1 2.6 2340   420 2.66 2 11.9 2 880 1 160 2.66 3 77.4 6 232 6 410 1.93

Example 8

[0165] Polymerization of Ethyl Acrylate in the Presence of A and ofD_(A) (10%)

[0166] The procedure of example 5 is repeated, in which 17.06 mg (0.070mmol) of bis-O-ethyl xanthate D_(A) are added to the stock solution (10mol % relative to xanthate A). The results are given in the table below:Conversion (%) M_(n) (g/mol) M_(n th) (g/mol)^(a) M_(w)/M_(n) 1 1.9 1960   360 2.39 2 10.1 3 100 1 020 2.35 3 25.7 4 800 2 270 2.01 4 71.8 5900 5 960 1.88

Example 9

[0167] Polymerization of Ethyl Acrylate in the Presence of A and D_(A)(20%)

[0168] Example 5 is repeated, in which 34.1 mg (0.14 mmol) ofbis-O-ethyl xanthate D_(A) are added to the stock solution (20 mol %relative to xanthate A). The results are given below: Conversion (%)M_(n) (g/mol) M_(n th) (g/mol)^(a) M_(w)/M_(n) 1 2 1 710 370 2.15 2 5.92 460 680 2.05 3 15.7 2 900 1 470   2.23

[0169] Based on the results described in examples 5 to 9, it is clearlyapparent that addition of D_(A) makes it possible to obtain a morecontrolled M_(n) evolution profile (closer to the theoretical profile),all the more so the greater the amount of D_(A) added. D_(A) activatesthe consumption of xanthate A and therefore the creation of the polymerchains generated by A.

Example 10

[0170] Polymerization of Styrene in the Presence of A:

[0171] 0.146 g (0.7 mmol) of A and 5.87 g (56.4 mmol) of styrene aredissolved in 6.45 ml of toluene. Fractions of this stock solution aredistributed in Carius tubes. The content of these tubes is degassed bythree successive cycles of “thawing-vacuum-return to ambienttemperature”. The tubes are then flame-sealed under vacuum. The tubesare then placed in a thermostatic bath at 110° C. They are withdrawn atregular time intervals, then opened and their content is analyzed.

[0172] The table below gives the evolution of the number-average molarmass (M_(n)) and of the polymolecularity index (M_(w)/M_(n)) as afunction of the monomer conversion. Conversion (%) M_(n) (g/mol)M_(n th) (g/mol)^(a) M_(w)/M_(n) 1 10.2 7 970 1 060 2.03 2 21 7 790 1960 2.09 3 34 8 030 3 040 2.05 4 49 8 250 4 290 1.97 5 76.3 8 150 6 5702.01

Example 11

[0173] Polymerization of Styrene in the Presence of A and of D_(A) (5%)

[0174] The procedure of example 10 is repeated, in which 8.53 mg (0.035mmol) of bis-O-ethyl xanthate D_(A) are added to the stock solution (5mol % relative to xanthate A). The results are given below: Conversion(%) M_(n) (g/mol) M_(n th) (g/mol)^(a) M_(w)/M_(n) 1 12.6 3 400 1 2603.05 2 45 5 400 3 960 2.73 3 48.8 6 400 4 280 2.44 4 92.5 8 100 7 9302.06

[0175] Based on the results described in examples 10 and 11, it isclearly apparent that addition of D_(A) makes it possible to obtain amore controlled M_(n) evolution profile (closer to the theoreticalprofile). Comparison of the tables of examples 10 and 11 for lowconversion rates shows that addition of 5% of D_(A) makes it possible toaccelerate the creation of the chains derived from A (the chains arecreated approximately twice as quickly based on the values for M_(n)).

Example 12

[0176] Polymerization of Ethyl Acrylate in the Presence of B

[0177] 0.190 g (0.69 mmol) of B, 3.4 mg (0.02 mmol) of AIBN and 5.5 g(55 mmol) of ethyl acrylate are dissolved in 6 ml of toluene. Fractionsof this stock solution are distributed into Carius tubes. The content ofthese tubes is degassed by three successive cycles of“thawing-vacuum-return to ambient temperature”. The tubes are thenflame-sealed under vacuum. The tubes are then placed in a thermostaticbath at 80° C. They are withdrawn at regular time intervals, then openedand their content is analyzed.

[0178] The table below gives the evolution of the number-average molarmass (M_(n)) and of the polymolecularity index (M_(w)/M_(n)) as afunction of the monomer conversion. Conversion (%) M_(n) (g/mol)M_(n th) (g/mol)^(a) M_(w)/M_(n) 1 4.2 2 790   610 1.98 2 11 2 940 1 1602.00 3 25.7 3 600 2 330 1.81

Example 13

[0179] Polymerization of Ethyl Acrylate in the Presence of B and ofD_(B) (5%)

[0180] The procedure of example 12 is repeated, in which 12 mg (0.034mmol) of bis-O-ethyl xanthate D_(B) are added to the stock solution (5mol % relative to xanthate B). The results are given in the table below:Conversion (%) M_(n) (g/mol) M_(n th) (g/mol)^(a) M_(w)/M_(n) 1 0.9 1830 350 1.78 2 1.3 1 800 380 1.87 3 4.1 1 960 605 1.96

[0181] Based on the results described in examples 12 and 13, it isclearly apparent that addition of D_(B) makes it possible to obtain amore controlled M_(n) evolution profile (closer to the theoreticalprofile). For a comparable conversion rate (approximately 4%), M_(n) iscloser to the theoretical values in the presence of D_(B).

1. A method for preparing a first generation polymer, which comprises astep of free-radical polymerization of a composition comprising: atleast one ethylenically unsaturated monomer, a source of free radicals,at least one compound (I) of general formula (IA) or (IB):

a compound of formula (II)

in which R¹ represents a group chosen from alkyl, acyl, aryl, aralkyl,alkene or alkyne groups, saturated, unsaturated or aromatic carbonaceousrings or heterocycles, or a polymer chain, Z and Z₁, which may beidentical or different, represent a group chosen from alkyl, acyl, aryl,aralkyl, alkene or alkyne groups, or saturated, unsaturated or aromaticcarbonaceous rings or heterocycles, which may be substituted, the group—OR² in which R² is an alkyl, acyl, aryl, aralkyl, alkene or alkynegroup, a saturated, unsaturated or aromatic carbonaceous ring orheterocycle, a polymer chain, —CH₂C_(n)F_(2n+1) with n between 1 and 20,or a group —CR⁵R⁶PO(OR⁷)₂ in which R⁵ and R⁶ are each separately ahydrogen atom, a halogen, an alkyl group, a heterocyclic group, a group—NO₂, —SO₃R⁸, —NCO, CN, R⁸, —OR⁸, —SR⁸, —NR⁸ ₂, —COOR⁸, O₂CR⁸, —CONR⁸ ₂,—NCOR⁸ ₂, or C_(n)F_(2n+1) with n between 1 and 20, R⁸, which may beidentical or different, being chosen from a group consisting of thefollowing groups: alkyl, alkenyl, alkynyl, cycloalkenyl, cycloalkynyl,aryl, optionally condensed with an aromatic or nonaromatic heterocycle,alkaryl, aralkyl and heteroaryl, R⁸ possibly being substituted with oneor more groups, which may be identical or different, chosen fromhalogen, ═O, ═S, OH, alkoxy, SH, thioalkoxy, NH₂, mono- or dialkylamino,CN, COOH, ester, amide, and C_(n)F_(2n+1) with n between 1 and 20,and/or optionally interrupted with one or more atoms chosen from O, S, Nand P, or R⁵ and R⁶ form, together with the carbon atom to which theyare attached, a group ═O or ═S or a hydrocarbon-based ring or aheterocycle, and R⁷, which may be identical or different, represents agroup R⁸ as defined above or they together form a C₂-C₄hydrocarbon-based chain optionally interrupted with a hetero atom chosenfrom O, S and N; the group —NR³R⁴ in which R³ and R⁴, which may beidentical or different, are chosen from optionally substituted alkyl,acyl, aryl, aralkyl, alkene or alkyne groups, and saturated, unsaturatedor aromatic carbonaceous rings or heterocycles, which may besubstituted, and R³ and R⁴ together form an optionally substituted ringcontaining at least 5 members, with the additional condition that R³ andR⁴ induce a delocalizing or electron-withdrawing effect with respect tothe electron density of the nitrogen atom, p is an integer greater thanor equal to
 1. 2. The method as claimed in claim 1, in which the amountof compound (II) is between 0.1 and 20 mol % relative to the number ofmoles of compound (I).
 3. The method as claimed in either one of claims1 and 2, in which Z is chosen from alkyl, haloalkyl, phenyl, alkene andalkyne groups.
 4. The method as claimed in either one of claims 1 and 2,in which Z is —OR² in which R² is an alkyl group.
 5. The method asclaimed in either one of claims 1 and 2, in which Z is —NR³R⁴ in whichR³ and R⁴, which may be identical or different, are an alkyl group. 6.The method as claimed in any one of claims 1 to 5, in which R¹ is asubstituted alkyl group.
 7. The method as claimed in any one of claims 1to 6, in which the transfer constant of the compound (I) with respect tothe monomer is less than
 10. 8. The method as claimed in claim 7, inwhich the transfer constant for the compound (I) with respect to themonomer is less than
 1. 9. The method as claimed in any one of claims 1to 8, in which the ethylenically unsaturated monomer(s) correspond(s) tothe formula CXX′(═CV—CV′) _(b)═CH₂, in which: V and V′, which may beidentical or different, represent: a hydrogen atom, an alkyl group or ahalogen, X and X′, which may be identical or different, represent H, ahalogen or a group R⁴, OR⁴, O₂COR⁴, NHCOH, OH, NH₂, NHR⁴, N(R⁴)₂,(R⁴)₂N⁺O⁻, NHCOR⁴, CO₂H, CO₂R⁴, CN, CONH₂, CONHR⁴ or CON(R⁴)₂, in whichR⁴ is chosen from alkyl, aryl, aralkyl, alkaryl, alkene or organosilylgroups which are optionally perfluorinated and optionally substitutedwith one or more carboxy, epoxy, hydroxyl, alkoxy, amino, halogen orsulfonic groups, b is 0 or
 1. 10. A polymer composition which can beobtained using the method as defined in any one of claims 1 to
 9. 11. Amethod for preparing an Nth generation block copolymer by free-radicalpolymerization, N being greater than or equal to 2, which comprises: afirst step of free-radical polymerization so as to form a firstgeneration polymer using a composition comprising: at least oneethylenically unsaturated monomer, a source of free radicals, at leastone compound (I) of general formula (IA) or (IB), a number N-1 offree-radical polymerization steps, each of these steps being carried outusing a composition comprising: at least one ethylenically unsaturatedmonomer, a source of free radicals, and the polymer obtained in thepreceding polymerization step, the ethylenically unsaturated monomer(s)being such that the block formed in this step is different in naturefrom the block formed in the preceding step, and the firstpolymerization step and/or the subsequent polymerization steps arecarried out in the presence of at least one compound of formula (II),the compounds (I) of formulae (IA), (IB) and the compounds (II) being asdefined in any one of claims 1 to
 9. 12. The method as claimed in claim11, in which the first polymerization step is carried out in thepresence of the compound (II).
 13. The method as claimed in claim 11 or12, in which at least one of the N-1 steps is carried out in thepresence of the compound (II).
 14. The method as claimed in claims 11,12 or 13, for preparing a second generation block copolymer, whichcomprises the free-radical polymerization of a composition comprising:at least one ethylenically unsaturated monomer, a source of freeradicals, and the first generation polymer.
 15. The method as claimed inany one of the preceding claims 1 to 14, in which the ethylenicallyunsaturated monomer is chosen from styrene or derivatives thereof,dienes, (meth)acrylic esters, vinyl nitrites and vinyl esters.
 16. Themethod as claimed in claim 14, in which the second generation blockcopolymer comprises two blocks chosen from the following combinations:polystyrene/poly(methyl acrylate), polystyrene/poly(ethyl acrylate),polystyrene/poly(tert-butyl acrylate), poly(ethyl acrylate)/poly(vinylacetate), poly(butyl acrylate)/poly(vinyl acetate), poly(ethylacrylate)/poly(tert-butyl acrylate), poly(tert-butylacrylate)/poly(vinyl acetate), poly(ethyl acrylate)/poly(butylacrylate), poly(butyl acrylate)/poly(vinyl alcohol), poly(acrylicacid)/poly(vinyl alcohol).
 17. The method as claimed in claim 14, inwhich the second generation copolymer comprises at least one blockconsisting of a random polymer obtained using a mixture of ethylenicallyunsaturated monomers.
 18. A polymer composition which can be obtainedusing the method as defined in any one of claims 11 to 17.